Powder metallurgical alloy



Unite States Patent 3,285,736 POWDER METALLURGICAL ALLOY Walter K. Brinn, Euclid, and Ronald L. Barto, Wicklilfe,

Ohio, assiguors to General Electric Company, a corporation of New York No Drawing. Filed Aug. 27, 1964, Ser. No. 392,649 3 Claims. (Cl. 75-122.5)

This invention relates to powder-metallurgical molybdenum-base alloys, and, more particularly, to such alloys strengthened both by solid solution and dispersion hardening so as to provide an alloy having high strength at elevated temperatures and good workability at room temperature.

In satisfying requirements for refractory metal alloys, arc-casting is often used to produce complex, mu-lti-phase alloys that can be heat-treated in combination with deformation processing to give materials having good adaptability, the structure and properties of which can be altered during manufacture to meet the end requirements. Alloys produced by powder metallurgy, on the other hand, have in the past been considered to be much less amenable to thermal and mechanical treatments to vary their proper ties. Problems of purity and difliculties in obtaining homogeneity in alloys produced from powder, rather than by melting, limited the availability of powder-metallurgical refractory metal alloys except for specialized purposes such as filament and support wire for lamps and electronic tubes;

Conversely, the cost-s involved in producing arc-cast refractory metal alloys are generally several times those necessary for the production of refractory metal alloys by powder-metallurgical methods, provided useful powder alloys could be designed and produced in a homogeneous, stable condition free from deleterious effects of impurities. It would be especially desirable from a cost standpoint to be able to provide reliable and reproducible powdermetallurgical alloys based on molybdenum and hardened by a combination of solid solution strengthening and dispersion hardening that would be workable at or near room temperature and have satisfactory ductility at room temperature and sufficient strength at elevated temperatures.

Many of the commercial arc-cast refractory metal alloys contain alloying additions such as zirconium and titanium which may form compounds such as carbides and oxides that are susceptible to reversible dissolution and precipitation in the matrix phase, thus allowing the titanium or zirconium additive to serve either as a solid solution strengthener or as a dispersoid or both in combination. It is readily apparent that thermal and mechanical treatments for such alloys cannot be consistently controlled to account merely for stress-relief and recrystallization, but also must account for partial or complete precipitation, dissolution, agglomeration and morphology of the precipitate. This is in strong contrast to the stability of properties that theoretically could be made available in insoluble dispersoid powder-metallurgical alloys. However, useful powder-metallurgical alloys of molybdenum have not heretofore been available having .suificiently desirable combinations of properties to satisfy certain of the demands of industry.

Accordingly, it is an object of the present invention to provide powder-metallurgical molybdenum-base alloys strengthened by both solid solution and dispersion hardening that have greatly improved strength at elevated temperatures and are workable at room temperature.

It is another object of the present invention to provide such alloys that are equivalent or superior in their properties to current commercial arc-cast refractory metal alloys.

Briefly stated, the present invention accomplishes the above-mentioned objects and others by providing a powdermetallurgical molybdenum-base alloy consisting essentially of, by weight, about 0.1l% thoria (T1102) and about 530% tungsten, the balance being molybdenum. Among the preferred and optimum ranges of compositions of molybdenumbase alloys within the invention are 0.1- 0.25% Th0 with 15-25% tungsten, and about 0.2% ThO with about 20% tungsten.

Alloys of the invention are made with conventional powder-metallurgical processing equipment. The desired quantities of tungsten, molybdenum and thoria powders can be blended together in a V or twin shell blender with an intensifier for about twenty minutes with the approximate particle sizes for the constituents being given in Fisher number as: molybdenum2.8, tungsten1.3, and thoria0.7. The blended powders may be packed in rubber containers having a rectangular cross section, sealed and pressed hydrostatically at a pressure of about 35,000, pounds per square inch. The green compacts, which are self-supporting after compaction, can then be sintered in a hydrogen furnace for about 15 hours at about 1850' C.

As is well known in the art, many types of processes can be used to convert such sintered billet into wrought products such as sheet or bar. As an example of one process for the production of sheet from sintered billet, the billet can be heated in a hydrogen atmosphere to 1200 C. and then rolled in drafts of preferably 1020% per pass to a total reduction of about 50% with reheating as necessary to keep the metal between 900 and 1200 C. The metal is then recrystallized and rolled to finish-gage sheet, preferably between 900 and 1200 C. at 10 20% per pass, followed by a stress-relief anneal. The amount of reduction per pas-s may depend on the capacity of the rolling mill.

Stress-relief annealing of the wrought product can be accomplished by heating in the temperature range of 900 C. to 1200 C. for times of about one hour. The wrought material can be recrystallized by heating in a temperature range of 1100 C. to 1400 C. for about one hour.

Table I shows the superiority of an alloy of the invention over unalloyed powder-metallurgical molybdenum in terms of increased recrystallization temperature and finer grain size as recrystallized. All material was warm rolled with a 92% reduction in thickness. Start and completion of recrystallization were detenmined for times of ten minutes at temperature in a hydrogen atmosphere furnace.

TABLE I [Recrystallization characteristics] Recrystallization F.) Grain Size Material (grains/mm!) Start Complete Unalloyed Mo 1, 740 2, 010 4, com, 500 M020W0.2 Th0; 2, 2, 600 8, 00010, 000

3 tion, and the transverse direction (trans) is perpendicu lar to the rolling direction but in the plane of the sheet.

" The Tinius-lsen cup depth is a measure of the form-- ability of the sheet, with a greater depth indicating greater workability.

.melting and recrystallization temperatures over unalloyed molybdenum also'have been achieved in these powdermetallurgical alloys without. losing the type of low temperatnre ductility and workability characteristic of the un- 5 alloyed metal.

Table III below presents a comparison of the properties of an alloy of theinvention containing about tungsten, 0.2% thoria, balance molybdenum, unalloyed: 'rnolybdenurn' produced by powder-metallurgical means and arc-cast molybdenum containing about 0.5% titanium, all in the form of.40 milthick'sheet. SR means.-

stress-relieved 1 hour at 1850 F. for the 20W'0.2% 2 T110 alloy and Mo0.5 Ti, whilethe pure molybdenum was stress-relieved at 1750 F. for 1 hour.

TABLE III [Comparative properties] It is intended that this invention be interpreted as broadlyas the true scope and spirit of the appended claims. I I

What we claim as new and desire to secure by Letters Patent of. the United States is: t

-1. .A molybdenum-base powder-metallurgical alloy workable at roomtemperature consisting essentially of by weight from about 0.1% to about 1% ThO- from about 5% to about tungsten, and thebalance molybdenum,

Tensile Strength (K s.i.)'

Temper- Alloy Condition at-ure 0.2% Yield Strength (K s.i.)

Percent Elongation in 1 inch ta zocsouomi o cameo: im'o'o Unalloyed M0 Mo-0.5 Ti

It can be seen from this table that the properties of the alloy of the invention are decidedly superior to those of unalloyed molybdenum, particularly at elevated temperatures, and that the alloy of the invention is roughly equivalent to or better than the considerably more expensive arc-cast alloys having 0.5% titanium.

The matrix phase of the alloy of the invention was shown to be homogeneous by anodization techniques known to the art.

The superior properties and economic improvements available through the use of alloys of the invention result in part from: (1) the strengthening of the matrix phase by solid solution hardening due to the addition of tung-' sten to the molybdenum; (2) the presence of a fine, uniform, insoluble, stable second phase, thoria, which contributes to the high temperature strength and resistance to grain growth at elevated temperatures; and (3) the fact that these two mechanims interacting provide their beneficial strengthening eifects while not destroying the room temperature workability of the alloys. The increases in References Cited by the Examiner A.E.C. bulletin AD-248201, Interim Report No. 5, July 1, 1960-August 31, 1960, Nelson et al., Table V and pp. T34-T36.

BENJAMIN R. PADGETT, Primary Examiner.

REUBEN EPSTEIN, Examiner.

M. I. SCOLNICK, Assistant Examiner. 

1. A MOLYBDENUM-BASE POWDER-METALLURGICAL ALLOY WORKABLE AT ROOM TEMPERATURE CONSISTING ESSENTIALLY OF BY WEIGHT FROM ABOUT 0.1% TO ABOUT 1% THO2, FROM ABOUT 5% TO ABOUT 30% TUNGSTEN, AND THE BALANCE MOLYBDENUM, THE POWDER-METALLURGICAL ALLOY STRUCTURAL CONSISTING OF PARTICLES OF THO2 DISPERSED IN A SUBSTANTIALLY HOMOGENEOUS TUNGSTEN-MOLYBEDNUM MATRIX PHASE. 